Defending champion Carlos Alcaraz won his first singles match in 139 days, while Aryna Sabalenka got her bid for a third straight US Open title off to a winning start on Monday to light up the second day of the year's final Grand Slam.
Former champion Iga Swiatek and last year's runner-up Amanda Anisimova also advanced with impressive victories while Stefanos Tsitsipas served up an upset by knocking out 10th seed Arthur Fils on a sunny afternoon in New York.
Alcaraz's condition was one of the tournament's biggest question marks after his long spell out due to a wrist injury but the Spaniard enjoyed a comfortable return to action with a 6-4 6-4 6-4 win over Russian Roman Safiullin.
"You don't know what's going to happen in the match after 4-1/2 months almost. You don't know what's going to happen in your body, in your mind, everything. It's totally different," Alcaraz told reporters.
"But I just tried to play one game at a time and tried to feel good, to enjoy playing points, feeling the love from the people. Hopefully they enjoyed it."
The second seed's win ensured some star power remained in the men's field.
Novak Djokovic's shock defeat on Sunday left another hole in a men's draw already without injured world number one Jannik Sinner.
SABALENKA, ANISIMOVA WIN EASY
Earlier at Arthur Ashe Stadium, Sabalenka looked on her way to ending a frustrating patch of poor form with a 6-2 6-4 win over Colombia's Camila Osorio that earned her a clash with Russian Polina Iatcenko in the second round.
"I'm happy with my performance," said Sabalenka, who has not reached a title match since triumphing at Indian Wells and Miami to complete the 'Sunshine Double' in March.
"She's a tough opponent, makes you work really hard to get every point."
American Anisimova, whom Sabalenka beat in the final last year, got her campaign off to a good start on her 25th birthday with a 6-1 6-3 win over compatriot Ashlyn Krueger at Louis Armstrong Stadium.
Greece's Tsitsipas upset Cincinnati winner Fils 4-6 7-6(3) 6-1 6-4 in an early match on the same court, booking a second-round meeting with South African Lloyd Harris.
"It's very easy to not feel confident, especially when you haven't beaten top players in a long time," Tsitsipas said. "I'm ecstatic about today. It's an important, good victory for me, for my confidence."
MUCHOVA, NOSKOVA GO THROUGH
Seven matches that were suspended on the outer courts on Sunday due to rain resumed on Monday, as organizers benefited from the extended three-day first-round schedule after expanding the main draw to 15 days in 2025.
Wimbledon finalist Karolina Muchova got the day underway on the Grandstand with a routine 6-3 6-0 win over Sinja Kraus and the Czech seventh seed plays Briton Katie Boulter next.
Linda Noskova, who defeated compatriot Muchova in the All England Club final, downed American Katie Volynets 7-5 6-1 to stay on track for back-to-back major titles.
It was a good day for the Americans on the largest of the outdoor courts. Emma Navarro ground out a 7-6(6) 6-7(1) 6-3 win over Frenchwoman Lois Boisson, while Taylor Townsend defeated Poland's Maja Chwalinska 6-2 6-3.
China's Olympic champion Zheng Qinwen, who was forced to go through qualifying only two years after reaching the quarter-finals in New York, beat Kristina Liutova 4-6 6-3 6-1.
Zheng's progress has stalled and her ranking has plummeted since an elbow injury forced her to undergo surgery last year.
Polish eighth seed Swiatek, the 2022 champion, beat China's Wang Xiyu 6-2 6-3 to kick off the night session on the second showcourt.
Another former winner, Stan Wawrinka, fell to Italy's Matteo Berrettini 7-6(4) 7-6(3) 6-0. The 41-year-old Swiss bid goodbye to the Grand Slam for the final time before his retirement at the end of the season.
Home hope Ben Shelton overcame Dutchman Tallon Griekspoor 1-6 6-1 7-6(3) 6-2 before Frances Tiafoe prevailed 6-4 4-6 3-6 6-3 6-4 in an all-American clash with Martin Damm.
Naomi Osaka unveiled another bold Grand Slam walk-on outfit before showing plenty of fight for a 7-6(6) 7-6(3) victory over unseeded Russian Anastasia Zakharova in a late-finishing match.
